'tis the Season for Auroras

Spring and Fall are good times to spot Northern Lights, and scientists
would like to know why.

October
26, 2001: Last Sunday, Oct. 21st, a cloud of magnetized gas
from the Sun (a "coronal mass ejection") swept past
Earth and rocked our planet's magnetic field. Northern sky watchers
were delighted as red and green lights rippled across the sky.
It was the aurora borealis -- breaking out for the third time
this month.

"The auroras were probably the most spectacular I have
ever witnessed," says Ryan Kramer, an observer in North
Dakota. "It was like being under a giant canopy. Northern
Lights filled the sky -- including directly above and even within
30 degrees of the southern horizon."

"I was amazed that the auroras were so bright to the
south of me," agreed Todd Carlson, who enjoyed the
spectacle from his home in Ontario, Canada. Indeed, before the
storm was done, observers as far south as the Carolinas in the
United States had caught a rare glimpse of Northern Lights.

"What an awesome display!" exclaimed Ronnie
Sherrill of Troutman, North Carolina, where the sky "exploded
into bright red and yellowish beams." The auroras were so
bright Sherrill and others saw them against an early evening
sky still lit by faint sunshine.

It was a good time to be outside.

Indeed, it may have been the best time: Autumn nights
are long and dark, but not yet wintry-cold -- a good combination
for sky watching. But there's more to it than that, say researchers.
Geomagnetic storms that ignite auroras actually happen more often
during the months around the equinoxes -- that is, early Autumn
and Spring.

It's a bit puzzling. Solar activity does not depend on Earth's
seasons. Why should geomagnetic storms?

"We've known about this seasonal effect for more than
100 years," says Dennis Gallagher, a space physicist at
the NASA Marshall Space Flight Center. "Some aspects of
it are understood, but not all."

Above: Still frames from a digital movie showing how
coronal mass ejections compress Earth's magnetosphere and trigger
auroras. Click
to view the full 750 kb Quicktime animation created by Digital
Radiance, Inc.

Geomagnetic storms erupt when solar wind gusts or coronal
mass ejections (CMEs) hit Earth's magnetosphere -- a magnetic
bubble around our planet that protects us from the relentless
solar wind. The magnetosphere is filled with electrons and protons.
Normally these particles are trapped
by lines of force (so-called "magnetic bottles") that
prevent them from escaping to space or descending to the planet
below.

"When a CME hits the magnetosphere," explains Tony
Lui, "the impact knocks loose some of those trapped particles.
They rain down on Earth's atmosphere and cause the air to glow
where they hit." Lui is a space physicist at the Johns Hopkins
University Applied Physics Lab.

"Precipitating particles mostly
follow magnetic field lines that lead to Earth's poles,"
he added. "The auroral ovals (circular regions of auroral
light around the magnetic poles) expand during magnetic storms."
Sometimes they grow so large that people at middle latitudes
-- like North Carolinans -- can see the light.

Such widespread storms are usually nurtured by what scientists
call "Bz" (pronounced "Bee sub Zee")
-- in other words, the component of the interplanetary magnetic
field (IMF)
that lies along Earth's magnetic axis. At the magnetopause, the
part of our planet's magnetosphere that fends off the solar wind,
Earth's magnetic field points north. If the IMF tilts south (i.e.,
Bz becomes large and negative) it can partially cancel
Earth's magnetic field at the point of contact.

"At such times the two fields (Earth's and the IMF) link
up," says Christopher Russell, a Professor of Geophysics
and Space Physics at UCLA. "You can then follow a magnetic
field line from Earth directly into the solar wind." South-pointing
Bz's open a door through which energy from the solar
wind can reach Earth's inner magnetosphere.

In the early 1970's Russell and colleague R. L. McPherron
recognized a connection between Bz and Earth's changing
seasons: The average size of Bz is greatest each year
in early Spring and Autumn.

It's
a result of geometry, explains Russell. The interplanetary magnetic
field comes from the Sun; it's carried outward from our star
by the solar wind. Because the Sun rotates (once every 27 days)
the IMF has a spiral shape -- named the "Parker spiral"
after the scientist who first described it. Earth's magnetic
dipole axis is most closely aligned with the Parker spiral in
April and October. As a result, southward (and northward) excursions
of Bz are greatest then.

Right: Steve Suess (NASA/MSFC) prepared this figure,
which shows the Sun's spiraling magnetic field from a vantage
point ~100 AU from the Sun.

"We've learned in the last 28 years that the north-south
component of the IMF controls the energy flow of the solar wind
into our magnetosphere," says Russell. Northward fields
have little effect, he added, but southward Bz's can
set the stage for substantial geomagnetic activity.

This week was a good example. The widespread auroral storm
of Oct. 21st and 22nd was preceded by a 24-hour period of mostly
south-pointing Bz. The IMF continued to tilt south
after a coronal mass ejection struck Earth's magnetosphere on
the 21st; and the ensuing
display of Northern Lights was one of the most memorable
of the current solar cycle.

The influence of Bz on geomagnetic activity is
undeniable, but researchers agree it's not the only influence.
For instance: The Sun's rotation axis is tilted 7 degrees with
respect to the plane of Earth's orbit. Because the solar wind
blows more rapidly from the Sun's poles than from its equator,
the average speed of particles buffeting Earth's magnetosphere
waxes and wanes every six months. The solar wind speed is greatest
-- by about 50 km/s, on average -- around Sept. 5th and March
5th when Earth lies at its highest heliographic latitude.

Left:
On October 11, 2001, during another autumnal geomagnetic storm,
Jody Majko captured this photo of bright Northern Lights above
the city lights of Winnipeg, Manitoba, Canada. [more]

In a recent Geophysical Research Letter (28, 2353-2356,
June15) Lyatsky et al argue that neither Bz
nor the solar wind can fully explain the seasonal behavior of
geomagnetic storms. According to their study, those factors together
contribute only about one-third of the observed semiannual variation.

What remains is a puzzle that space scientists are still trying
to solve. "This is an area of active research," notes
Lui. "We don't have all the answers yet, because it's a
complicated problem."